An improved process is provided to fade denim fabric employing chemical treatment providing a decreased chemical oxygen demand.
Denim garments such as slacks, jackets and skirts are considered by many to be more fashionable once they have attained a faded, worn appearance. Accordingly, denim fabrics and garments are frequently subjected to a bleaching procedure during their manufacture to give them a bleached, superbleached, rifled or whitewashed appearance. While such prebleached goods are a very marketable product, the bleaching procedures conventionally employed are relatively labor intensive, which adds significantly to the cost of the bleaching process. U.S. Pat. No. 4,218,220 discloses that it is sometimes desirable to prepare prefaded denim garments uniformly faded, that is prefaded blue jeans free of unwanted streaks. Satisfactory, unstreaked, suitably faded blue jeans were hitherto obtained only by repeated washings. The patent teaches subjecting the denim fabric to a washing cycle comprising an initial wash with detergent and emulsifier, a suitable intermediate rinsing operation, a bleaching operation in which the garments are subjected to the simultaneous action of bleach and a fabric softener of the quaternary ammonium type, alone or with the addition of a suitable amount of detergent, a further rinsing operation, and an optional final treatment with fabric softener and laundry sour. The patent teaches the use of a chlorine bleach, such as, sodium hypochlorite or trichloroisocyanuric acid or the like as a bleach. U.S. Pat. No. 4,852,990 teaches a modification wherein denim garments first are desized, then contacted with an aqueous polyacrylic acid solution. A chlorine-type bleaching agent is subsequently added to provide a uniform bleached appearance.
Subsequently, the trend has been toward a look featuring random faded effects.
One such manifestation of this trend is the practice of stone-washing - that is, immersing cloth in water containing no other substance than pumice stones. The effect it is sought to produce on denim treated by this method is one of natural fading, a "used" look characterized by the contrast between light and dark areas; in made-up garments however, the effect tends to appear on and around the seams only, whereas the color of the remaining fabric remains substantially uniform.
U.S. Pat. No. 4,740,213 discloses a process in which granules of a coarse, permeable material, such as pumice, are impregnated with a chlorine bleaching agent tumbled in a drum with denim fabric in a dry state. Subsequent traces of the chlorine bleaching agent are removed, optionally by an antichlor such as acidic hydrogen peroxide.
However, chlorine bleaching agents are known to be very destructive to cotton, consequently alternative bleaching agents have been employed to produce the faded look. Potassium permanganate is very desirable for such an oxidative treatment. When applied in a solution an even fading is obtained, and when impregnated into an inert porous material it provides a desired random uneven oxidation of colorbodies when tumbled with fabric ("rocking"). Unfortunately, dark colored, insoluble manganese dioxide is deposited on the denim resulting in a dirty, stained appearance. The manganese dioxide can be removed by a process called "neutralizing", that is, reducing the manganese dioxide to soluble manganous salts, usually by sulfites, thiosulfate, hydroxylamine and the like. These reducing agents must be used in a large excess and at a pH of 2.5 to 3.0 causing damage to the cotton fibers. The excess reducing agent from the neutralizing step is very undesirable to dispose of because of its very high toxicity and high chemical oxygen demand.
After neutralizing the denim is frequently "brightened" or bleached to enhance the contrast between the dyed and the decolorized areas. Currently a hypochlorite bleach or a sodium perborate bleach bath is employed for brightening.